1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more specifically to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
Many such imaging systems are currently available, including those disclosed by applicants in various other issued patents and patent applications, including but not limited to U.S. patent application Ser. No. 10/434,374 and U.S. Pat. No. 6,064,517. The objective disclosed in the '374 application is a relatively small catadioptric objective exhibiting beneficial optical qualities. The small size catadioptric optical design of the '374 application exhibits generally good overall performance, particularly at high numerical apertures (NAs) over a relatively broad spectral range in the deep ultra-violet (DUV) region. However, chromatic correction can become increasingly difficult for light energy provided in the DUV region.
The objectives disclosed in the '517 patent are relatively large catadioptric objectives providing certain desirable optical qualities. The relatively large catadioptric optical designs in the '517 patent are generally corrected over a narrow spectral bandwidth and thus can be capable of performing ultra-high NA imaging over relatively large field sizes.
The small ultra-high NA designs presented in the '517 patent and the small broad band designs presented in the '374 application have certain similar advantageous qualities, including but not limited to their ability to be made from a single glass material. However, the lenses employed have different arrangements and are used in different ways to generally achieve different goals.
In using an inspection design for the applications discussed, it can be beneficial to improve the overall NA and field size of the system. Generally, the NA represents the range of angles for which light can be delivered to and collected from a specimen using the design. The field size is the diameter at the specimen location over which all the optical performance requirements are satisfied. For example, certain systems may successfully and efficiently employ ultra-high NA imaging over large field sizes. In this specific context, ultra-high NA illumination and imaging angles may include, but are not limited to, angles up to 76 degrees. Generally, previous designs have limitations of lower NAs, smaller field sizes, larger optical element diameters, or very tight manufacturing tolerances (i.e., lenses must be precisely crafted) in the high end applications discussed. All of these limitations can adversely affect overall performance and cost of the system.
Based on the optical characteristics desirable in such a design, an ultra-high NA and large field can offer improved capabilities when inspecting specimens using light energy in the DUV range. For example, but not by way of limitation, operation at wavelengths of 193, 213, 244, 257, 266, 325, 355 or greater up through visible wavelengths may result in beneficial performance in certain circumstances. In addition, relatively small size and relatively loose manufacturing tolerances can provide advantages over previous designs.
It would therefore be beneficial to provide a system and objective for use in conjunction with standard microscopes and microscopy applications that overcome the foregoing drawbacks present in previously known imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.